The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with surface coils of medical imaging equipment for receiving electromagnetic signals from resonating nuclei and will be described with particular reference thereto. It is to be appreciated, however, that the invention may also find utility in other magnetic resonance applications, such as well logging, chemical analysis, and the like.
Heretofore, various types of coils have been positioned to receive electromagnetic signals for magnetic resonance imaging and spectroscopy, particularly whole body, body portion, and localized coils. The whole body and body portion receiving coils had standard sizes which were selected for readily comparing the patient's whole body or a selected body portion. Due to the standardized coil size and variable patient size, a significant void or empty region was defined between the coil and the portion of the patient to be imaged.
Localized or surface coils were configured from rigid non-conducted sheets of plastic or nylon on which wire or other conductors were mounted. The flat coils were constructed in a variety of sizes and a variety of winding patterns to facilitate positioning the coil adjacent a selected area of the patient and imaging the selected patient area. By placing the localized coil in direct contact with the patient, the intervening air gap along with air gap associated signal-to-noise degradation and aliasing were eliminated or reduced.
A common surface coil configuration was a single loop of wire. To receive signals from deeper within the patient, larger diameter loops were utilized. The depth of the coil's region of sensitivity can also be adjusted with more complex winding patterns, such as a planar arrangement of concentric loops with current flows in different directions.
At high frequencies, the coils interacted strongly with the subject. The coils were most sensitive to the regions of the subject which were in immediate proximity to coil segments. Accordingly, increasing the complexity of the coil winding placed additional coil segments adjacent the subject, each of which was most sensitive to the immediately continguous region. The more windings in the coil pattern, the more aggravated the problems attributable to the very high sensitivity near the conductors became.
A configuration with two planar concentric loops tended to lower the sensitivity of the coil center close to the coil but gave better uniformity with depth into the subject along the coil axis. However, it increased sensitivity to surface material between the inner and outer loops. This increase interloop sensitivity tended to counterbalance the reduction in surface sensitivity at the center of the coil leading to a negligible, if any, improvement in the signal-to-noise.
In accordance with the present invention, a localized coil is provided which overcomes the above referenced problems and others, yet focuses more specifically on a selected region of interest lying deeper within the subject.